Fibre optical sensor technique is a field that during the last ten years has been predicted a very expanding market. The expansion has, however, not reached the pace that was predicted. The reason for this is principally the difficulty to separate the sensor information from the varying influence from the environment, both on the sensor and on the transmission to and from the sensors. Another reason is that the single-mode technique that has the highest degree of sensitivity is complex and demands that the components used are very accurate. The multimode technique, on the other hand, can allow a more simple design and moreover, it uses more robust components. Thereby it is possible to reach such a level for the costs for the fibre optical sensors that they become economically interesting. But the multimode technique has also not experienced a break-through, mostly due to the lower sensitivity that hitherto has been achieved with this technique. Conventional multimode technique is of cause based on principles of intensity modulation of the light that is transmitted in the core of the multimode fibre. This technique is less sensitive than the phase-sensitive detection that the single-mode technique is based on.
The fibre optical sensors have potentially very great advantages, primarily due to their non-galvanic construction, that result in for instance insensibility for EMP, EMC etc. The sensitivity that can be achieved is also as good as or better than what can be achieved with other types of sensors. Another interesting characteristic of the fibre optical sensor technique is the possibility to multiplex several sensors along the same fibre optical cable and in that way implement sensor systems that is able to cover for instance large surveillance zones. The sensors of the system can be dimensioned to be sensitive to several physical quantities such as pressure and magnetism. Great tactical advantages can be achieved by this. In order to achieve this goal the sensors have to be based upon such a technique that a realization of the sensor technique is robust and functionally reliable and this at a reasonable cost in comparison with conventional technique.